Boost Carousel boost count is wrong

**Describe the bug**
Frequently, the number of boosts that a carousel claims to have is more than the number of boosts actually in the carousel. The carousel will say that there are 14 boosts, but only 13 boosts will be in the carousel.
Site: Phanpy.social
Instance: mstdn.social

**To Reproduce**
Scroll to a boost carousel. Look at the number of boosts in the upper left. Scroll through the boosts. Sometimes (about half of the time for me) the number of boosts in the carousel is lower than the number claimed.

**Expected behavior**
The number in the upper left of the carousel should be the same as the number of boosts actually in the carousel.

**Screenshots**
As you can see, the carousel claims to have 14 boosts. I've scrolled to the last boost (the right arrow is dimmed), and its the 12th boosts. Either 2 boosts are missing, or the number in the upper left should say 12.
If applicable, add screenshots to help explain your problem.

**Desktop (please complete the following information):**
- OS: Chromeos
- Browser chrome
- Version 121

**Additional context**
I have 3 columns setup. Main, Notification, and a List

Start by reproducing the issue on Phanpy.social with a carousel, scrolling to its final boost and comparing that item count with the number shown in the upper-left. Done means the displayed count matches the boosts that can actually be viewed in the carousel, including with the reported three-column setup.

Written by the indexing model from the issue text.